1. Field of the Invention
The present invention relates to a leakage flux-type power conversion transformer which is used in a resonance inverter of a cold cathode tube illumination device, and in resonance converters such as a switching power supply and a non-contact charger. More specifically, the invention relates to the constitution of a power conversion transformer having a comparatively large output which uses braided wires as windings.
2. Description of the Related Art
As many types of electrical devices are being miniaturized, there is a strong demand to improve the power apparatuses for these devices by miniaturizing them, increasing their frequency and power conversion efficiency, reducing their noise levels, and so on. A resonance circuit system is generally used to achieve this. The resonance circuit system employs a method of zero bolt switching or zero current switching, which are characterized by high efficiency and low noise levels. The resonance circuit system mainly uses leakage inductance of a power conversion transformer, and provides a wide gap between the magnetic circuits of the primary winding and the secondary winding. (Hereinafter, a power conversion transformer having such a magnetic circuit constitution will be termed a xe2x80x9cleakage flux-type power conversion transformerxe2x80x9d.)
In a power conversion transformer for a non-contact charger which transmits power by electromagnetic induction, a primary winding is provided on the charger side and a secondary winding is provided on the cordless device side. Since the primary winding and secondary winding are separated for functional reasons, there is a wide gap between their magnetic circuits, obtaining a leakage flux-type power conversion transformer. For this reason, the resonance circuit system is also used in the non-contact charger. A resonance converter is constituted by combining the leakage inductance element of the power conversion transformer with a capacitor, and power is transmitted from the primary winding to the secondary winding by high-frequency oscillation.
FIG. 5 shows one example of a power conversion transformer for a non-contact charger. Reference code 10 represents the transmitting side, and reference code 20 represents the receiving side (cordless device side). The two bobbins 12 of the transmitting side 10 comprise winding axes 12a which are cylindrical in cross-section, as shown in FIG. 6. A primary winding 13 is wound around each of the winding axes 12a. The lead wires of the primary winding 13 are connected to terminals 11, attached to the bobbins 12. The two primary windings 13 are connected in series via the terminals 11 and an un-illustrated conductive pattern of a printed substrate. Reference code 15 represents a core comprising a U-shaped magnetic substance having two legs 15a. The two bobbins 12 are secured to the core 15 by inserting the legs 15a into holes in the winding axes 12a. 
The receiving side 20 has a similar constitution. Primary windings 23 are wound around winding axes 22a of two bobbins 22, which terminals 21 are attached to. The two primary windings 23 are connected in series via the terminals 21 and an un-illustrated conductive pattern of a printed substrate. Reference code 25 represents a core comprising a U-shaped magnetic substance having two legs 25a. The two bobbins 22 are secured to the core 25 by inserting the legs 25a into holes in the winding axes 22a. 
Litz (Litzendraht) wire is made by bundling together and twisting insulated single wires (hereinafter termed xe2x80x9cstrandsxe2x80x9d), and is used as the wire material for the windings in the above power conversion transformer. By using Litzwire, it is possible to reduce the skin effect, whereby current density deviates toward the surface of the strand as a result of the magnetic field generated by its own high-frequency current, and eddy current loss known as proximity effect, which is caused by magnetic flux leaked from other strands. However, when used in a power conversion transformer which requires a comparatively large output, the Litz wire cannot be not sufficiently twisted due to the great number of strands it comprises. When this type of Litz wire is used as the winding wire of the leakage flux-type power conversion transformer, some strands are wound near the surface of the core and some are wound near the gap, leading to variation in the inductance values of the strands. As a result of electromagnetic coupling between the strands, when a high frequency current is passed through them, the current concentrates in the strands having smaller inductance, consequently increasing the winding loss and making it difficult to improve the efficiency of the transformer.
Accordingly, a braided wire may conceivably be used, since in a braided wire there is little positional deviation of the strands even when a great number of them are provided. FIG. 7 shows an example constitution of a braided wire 30, in which multiple cluster wires 31, each comprising multiple strands 31a arranged in a horizontal row, are braided together. The cluster wires 31 of the braided wire 30 are braided together while changing their positions evenly in the up, down, left, and right directions. Therefore, the positions of the cluster wires 31 and the strands 31a deviate much less than in a Litz wire. As a result, there is less variation in the inductance values of the strands 31a. Increase in the winding loss, caused by the concentration of current in the strands with smaller inductance, is thereby reduced.
However, the conventional braided wire 30 is braided in a spiral at a narrow braid pitch, such as in the external shield wire section of a coaxial cable. Since the length of the strands 31a greatly exceeds the actual length of the braided wire 30, the direct current resistance is greater than in a Litz wire. Further, since the width of a braided wire having a narrow braid pitch increases, when multiple wires are wound around the winding axis, the portions were the wires overlap each other increases. At this time, the magnetic flux interlinkage increases as far as the inner layer, producing a strong skin effect and increasing the winding resistance at high frequencies. Since the direct current resistance and the winding resistance at high frequencies increase, it has not been possible to obtain a power conversion transformer with sufficient conversion efficiency by using the conventional braided wire 30.
The leakage flux-type power conversion transformer of the present invention comprises a first core having a leg; a second core having a leg; a primary winding, which is wound around the leg of the first core; and a secondary winding, which is wound around the leg of the second core. The primary winding and the secondary winding are electro-magnetically coupled together. At least one gap is provided between the leg of the first core and the leg of the second core. At least one of the primary winding and the secondary winding comprises a braided wire, which is braided from three or more cluster wires, each comprising a plurality of wire strands. The size of the braid pitch (P) of the braided wire is such that the ratio (W/P) between the average one-turn winding length (W) of the strands and the braid pitch (P) is between 0.5 and 2.5.